An unexpected 27-20 loss to the Chargers on Thursday night made Peyton Manning and the Broncos look average and placed all their regular-season goals in jeopardy.

“It humbles us and gets us back to the drawing board,” Denver defensive end Shaun Phillips said.

Once in the best position to capture the AFC West and home-field advantage through the playoffs, the Broncos (11-3) aren’t a sure thing for either anymore.

(credit: Evan Semón/CBS4)

They have as many losses as their main competition in both races – Kansas City for the division and New England for home field.

Denver holds the tiebreaker against the Chiefs, but not the Patriots.

The Broncos finish the season on the road against Houston and Oakland, two teams with a combined record of 6-20. But after a home loss to San Diego (7-7), nothing feels like a sure thing anymore – even for the team that still averages 38 points despite its lowest output of the season.

Over a stretch that began late in the first quarter and lasted nearly half the game, the Broncos’ offense ran 13 plays and gained 13 yards.

“We didn’t play well, didn’t stay on the field, didn’t have the ball much and, when we did, we didn’t do much with it,” Manning said.

Five things we learned from San Diego’s upset over the Broncos:

MANNING IS STOPPABLE: It helped that he wasn’t on the field much. It helped that Wes Welker wasn’t available. It helped that San Diego’s coach, Mike McCoy, was Manning’s offensive coordinator last year and knows how the quarterback’s mind works as well as anyone. Still, Manning has shredded up the best-laid plans of pretty much every team he’s played this year. The thought that anyone could hold Denver’s 2013 offense to two touchdowns and 20 points seemed almost laughable. Until now. It’s highly likely that New England, Cincinnati and other possible playoff opponents will be devouring this game tape.

DEFENSE STILL NEEDS WORK: This wasn’t a good defense coming into December and the tinkering the Broncos have been doing over the last two games isn’t helping. A neck injury that has harmed his performance is costing linebacker Wesley Woodyard playing time in favor of Paris Lenon, a 12th-year veteran the team signed off the street at the end of the preseason. Safety Duke Ihenacho has lost his starting spot toOmar Bolden. Rookie cornerback Kayvon Webster continues to get schooled while playing for Champ Bailey. What the Broncos need is for Bailey to get healthy, for Von Miller to start showing up on a consistent basis and for someone, anyone, on this dinged-up defense to start making big plays.

REVVED-UP RIVERS: Rivers is better than Manning, better than Tom Brady, better than even Aaron Rodgers – in December, that is. The San Diego quarterback improved his league-best record to 28-6 when the calendar flips to the final month of the year. And while his stats Thursday weren’t overwhelming – 12 of 20 for 166 yards and two touchdowns – he did help the Chargers nearly double Denver in time of possession. “(Rivers) is an elite quarterback in this league,” Denver defensive lineman Terrance Knighton said. “They didn’t try to make the big plays, just took what we gave them. Hitting us for four yards here, eight yards there, 12 yards there. They kept the ball out of our offense’s hands.”

BACK IN CONTENTION: Just like that, the Chargers are a legitimate factor in the playoff chase. They have as many wins as Miami and Baltimore, who are also in the running for the AFC’s last playoff spot. “We always seem to make it interesting,” Rivers said. “We’re a .500 ball club that’s going to fight like crazy to get the next two.” San Diego finishes the season with home games against Oakland and Kansas City. But coach Mike McCoy isn’t thinking that far ahead. “We can’t control the rest of the league; all we can do here … is play our best football week in and week out,” McCoy said.

TAKING HEART: Though it’s hard to find positives in a loss, the Broncos can sleep a little easier knowing this stat: The last four Super Bowl champions – and five of the last seven – lost in Week 15. Not a bad bit of knowledge for a team that headed into the playoffs last year on an 11-game winning streak, then promptly lost.